Radio frequency identification (RFID) systems are used in a wide variety of applications, and provide convenient mechanisms for the tracking, identification, and authentication of persons or objects. An RFID system typically includes one or more readers deployed at selected locations in an installation. Readers are typically deployed where it is desired to control or to receive information about objects or persons bearing or associated with RFID tags. For example, readers may be deployed so as to cover entrances and exits, inventory control points, transaction terminals, and the like. Each reader is capable of receiving information from RFID tags, with each tag typically being associated with an object or person. A tag may be affixed to or embedded in an object with which it is associated, or be part of a badge, card, or token given to a person. Signals conveyed between the tag and the reader allow the reader to sense information on the tag. This information may include, for example, authentication or identification information, or may include instructions, such as a sequence of processes or operations to be conducted upon an object bearing the tag.
Each tag may include stored information that is communicated wirelessly to the reader. Tags typically carry information in onboard memory such as ROM, and the amount of information may range from a single bit to kilobits or even more. Single bit tags typically serve as surveillance devices, such as theft prevention tags. Information amounting to a few bits or tens of bits may serve as an identifier, such as may be found in a badge or smart card, while information amounting to kilobits may comprise a portable data file that can be used for identification, communication, or control. The reader may, for example, extract information from a tag and use it for identification, or may store it or convey it to a responsible party. Alternatively, a data file may include a set of instructions that may initiate or control processes or actions without recourse to, or in coordination with, information stored elsewhere.
A tag typically includes a wireless communication device, for example a transmitter or transponder, that is capable of wirelessly communicating stored information to the reader. The tag may communicate the information independently or in response to a signal, such as an interrogation signal, received from the reader. Both active and passive tags are known in the art. An active tag has an onboard power source, while a passive tag may operate without an internal power source, deriving its operating power from a field generated by the reader. Passive tags are much lighter and less expensive than active tags and may offer a virtually unlimited operational lifetime. However, passive tags typically have shorter read ranges than active tags and require a higher powered reader. Passive tags are also constrained in their capacity to store data and their ability to perform well in electromagnetically noisy environments.
Sensitivity and orientation performance may also be constrained by limitations on available power. Despite these limitations, passive transponders offer significant advantages because they have an almost indefinite lifetime and are generally less expensive than active transponders or transmitters.
A passive tag typically includes memory, which may be read only memory (ROM), nonvolatile programmable memory such as electrically erasable programmable read only memory (EEPROM), or random access memory (RAM), depending on the applications to which the tag is to be put. Programmable memory used by a passive tag should be nonvolatile, so that data is not lost when the tag is in a powered down state. When the tag is not actively communicating with the reader, the tag is in a powered down state.
One commonly used implementation of a passive RFID tag includes analog or digital circuitry for processing signals received from and sent to the reader, as well as a coil for communicating with a compatible reader, for example by inductive coupling. The coil is also often referred to as an antenna. Communication through inductive coupling typically involves superimposing the data upon a rhythmically varying field or carrier wave, that is, using the data to modulate the carrier wave. The carrier wave may suitably be a sinusoidal wave.
In order to receive data from a passive tag or transponder that communicates through inductive coupling, the reader generates a magnetic field, typically using a reader coil that inductively couples to the transponder coil. The magnetic field induces a voltage in the transponder coil, thereby supplying power to the transponder. Data may suitably be transmitted to the reader by changing one parameter of the transmitting field. This parameter may be amplitude, frequency or phase.
The passive tag communicates with the reader by changing the load on the transmitting field. Load changes may suitably affect either the amplitude or phase of the field. These changes to the field are sensed by the reader coil, which produces a modulated current in response to the field. This current is analyzed, for example, demodulated, to extract the data, which is then used in ways called for by the design of the particular RFID system.
Typical prior art readers may employ a single coil to generate the power RF field, transmit data, and receive data from the RFID transponder. If a single coil is used, it must be relatively precisely tuned. Typically, the coil used is a resonant inductor. The use of a very precisely tuned resonant inductor is relatively expensive. In addition, the operating range of typical prior art readers typically depends on the diameter of the reader coil, leading to a relatively short operating range.
There exists, therefore, a need for systems and techniques that will allow for a reader having a lower cost sensing circuit and a greater operating range.